Heat exchange system

ABSTRACT

The invention provides a method and apparatus for integrating the heat transfer zones of plate fin and tube and finned tube exchangers by providing a plurality of heat exchange circulation paths within a single heat exchange zone of a heat exchanger.

TECHNICAL FIELD

This invention pertains to the field of heat exchange and specifically to a more efficient design of heat exchangers.

BACKGROUND OF THE INVENTION

Plate fin and tube heat exchangers or externally finned tube exchangers have long been employed to recover process heat. These exchangers are most often employed to heat or cool a low density gas stream located on the finned side against a denser fluid with higher heat transfer coefficient within the tubes. The extended surface on the finned exterior pass allows greater heat transfer surface than a bare tube and provides greater heat transfer at a low-pressure drop.

The art has not heretofore recognized the unexpected advantage of using multiple process streams on the tube side of one or more heat exchangers and interlacing the heat transfer zones to allow more effective heat transfer was not recognized in the art. Kohlenberger U.S. Pat. Nos. 5,790,972 and 5,444,971 disclose inlet cooling of a gas turbine using zones in series, but do not suggest the advantages of integrating the fluid circuits into one heat exchange zone.

SUMMARY OF THE INVENTION

The invention may be described in several ways as alternate embodiments of the same novel discovery.

A method for operating a heat exchanger of the plate fin and tube or finned tube type that can be used in heat transfer system that comprises:

-   -   a. providing a first working fluid on the finned exterior side         of the heat transfer device,     -   b. providing two or more working fluids flowing in separate         circuits within the tube circuits of the heat transfer device,     -   c. feeding the first working fluid to the exterior finned side         of a heat transfer zone or zones to transfer heat to or from the         first working fluid thereby heating or cooling the first working         fluid to a higher or lower temperature,     -   d. feeding the second working fluid into a tube or group of         tubes to be heated or cooled by the first working fluid,     -   e. feeding a third or more working fluid(s) into a tube or group         of tubes to be heated or cooled by the first or second working         fluid.     -   f. the multiple tube circuits being interwoven to more         effectively approach the cooling curve of the finned external         side fluid.

In a preferred embodiment the invention provides:

-   -   g. a heat exchange device having one or more finned exterior         side working fluid streams against multiple tube side circuits,     -   h. the multiple tube circuits being interlaced to more         effectively approach the cooling curve of the finned exterior         side working fluid.

In a more preferred embodiment the invention further provides:

-   -   i. A method of circuitry of a plate fin and tube or finned tube         exchanger that allows for more effective heat transfer by         adjusting the cooling curve, by providing multiple working         fluids in separate but integrated circulation paths.     -   j. A method of construction and design allowing complex         circuitry of a plate finned and tube or finned tube exchanger         utilizing multiple tube side circuits that are interlaced to         accomplish more effective heat transfer than would be possible         with multiple tube side fluid streams arranged in series without         interlacing the circuitry.

The invention may be described as a method of recovering energy that comprises:

-   -   providing a multiple zone integrated plate fin and tube or         finned tube exchanger that can be used in heat transfer system         that comprises the steps of:     -   a. providing a first working fluid circulation path on a finned         exterior side of a heat transfer device,     -   b. feeding the first working fluid to the circulation path on         the exterior finned side of a heat transfer zone to transfer         heat to or from the first working fluid thereby heating or         cooling the first working fluid to a higher or lower         temperature,     -   c. feeding a second working fluid into a second circulation path         within the heat exchanger to be heated or cooled by the first         working fluid.     -   d. And feeding a third fluid into a circulation path to be         heated or cooled by the first working fluid or the second         working fluid.

Fluids of the same or different composition may be used.

An alternative embodiment is a method of construction of a heat exchanger that comprises providing a plurality of circulation paths in a plate fin and tube or finned tube exchanger utilizing a plurality of tube side circuits that are interlaced to accomplish more effective heat transfer than would be possible with a plurality of tube side fluid streams arranged in series without interlacing the circuitry.

As noted above first working fluid and the second working fluid have the same composition or the working fluids may be of different composition.

The invention may also be described as an energy recovery apparatus specially designed to practice the methods described above that comprises:

-   -   a. A finned surface heat exchanger comprising a plurality of         circulation pathways for a plurality of working fluids, and     -   b. circulation means to pass a plurality of working fluids into         at least one heat exchange zone. For example the apparatus may         comprise a plurality of working fluid streams on the tubing         interior side of a plate fin and tube or finned tube exchanger.         In a preferred embodiment the apparatus further comprises         multiple heat recovery stages to provide additional heat         recovery.

The inventive concept may also be embodied as a method for an energy recovery system for increasing the efficiency of a gas turbine exhaust heat recovery by providing an integrated tube side heating circuitry to heat a plurality of working fluid circuits while cooling the exhaust stream of the gas turbine.

In summary, the invention provides a system for more efficient heat transfer in a plate fin and tube or finned tube exchanger by providing a plurality of separate working fluids in a plurality of working fluid circulation paths.

The invention is illustrated by the specific example set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sketch of a typical integrated coil having two working fluids in separate circuits.

FIG. 2 is a sketch of an integrated coil having 4 working fluids with interwoven circuitry.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a method of interweaving streams with various working fluids in a common plate fin and tube or finned tube exchanger to accomplish more efficient heat transfer.

The invention may be employed to increase the hear recovery efficiency of any gas turbine or other device that produces a waste heat exhaust or is a source of process heat. Alternatively the arrangement may produce more efficient refrigeration at locations requiring refrigeration or cooling.

These examples are provided to illustrate the invention and not to limit the concepts embodied therein. The invention is defined and limited by the claims set out below. The examples below were modeled using a commercial process simulator.

EXAMPLE 1

In FIG. 1 a simple two fluid system is shown. The principals illustrated in the more complex system also apply to the two fluid system of FIG. 1. Turning to FIG. 2, a first working fluid consisting of 88.67 mol % N2 and 11.32 mol % H2O enters the heat exchanger at 263.2F, 366.5 psia, 18,946 moles/hr and leaves the exchanger at 101.9 F, 363.3 psia. It exchanges heat with a 2^(nd) working fluid, natural gas in this illustration, entering at 45.6 F, 512 psia, 4460 moles/hr and leaving at 241.5 F, 501 psia. The first working fluid also exchanges heat with a 3^(rd) working fluid, water, entering at 74.4 F, 61 psia, 17660 moles/hr and leaving at 209.5 F, 42.8 psia. The first working fluid also exchanges heat with a 4^(th) working fluid, water, entering at 92 F, 54.7 psia, 20,900 moles/hr and leaving at 107 F, 44.7 psia. The exchanger is modeled with fluid separation and removal after each heat exchange zone as can be accomplished with the invention device. Under the invention integrated heat exchange design, the total UA required is 934,180 BTU/F-hr. If working fluid streams 2, 3 and 4 are arranged in series without integrated heat exchange and with fluid separation and removal after each heat exchange zone, the total UA required to accomplish the same leaving working fluid 1 temperature is 1,127900 BTU/F-hr. In this example the flow-rate of the 4^(th) working fluid is increased from 20,900 moles/hr to 25,760 moles/hr. Since UA is the heat transfer coefficient times the required heat transfer area, one can assume the heat transfer coefficient is essentially the same. Therefore the invention integrated heat exchanger design using the interwoven heat transfer zones can achieve the same desired working fluid 1 exit temperature with 20.7% less heat transfer area.

The working fluids may be any available fluid that can be circulated at pressures within the design limits of the selected exchanger and will be selected depending on available fluids at the site. They may include multiple streams of the same fluid at different temperatures and pressures (such as water in example 1) or the fluids may be different (such as natural gas, working fluid 2 in the example). 

1. A method of recovering energy that comprises: Providing a multiple circulation path integrated plate fin and tube or finned tube heat exchanger that can be used in heat transfer system that comprises the steps of: a. providing a first working fluid circulation path on a finned exterior side of a heat transfer device, b. feeding the first working fluid to the circulation path on the exterior finned side of a heat transfer zone to transfer heat to or from the first working fluid thereby heating or cooling the first working fluid to a higher or lower temperature, and feeding a second working fluid into a second interior (tube side) circulation path within a heat exchanger to be heated or cooled by the first working fluid and c. providing a third fluid circulation path in the heat exchanger and feeding a third working fluid into the third interior (tube side) circulation path to be heated or cooled by the first working fluid. d. interweaving the second and third fluid flow paths of the exchanger to achieve more efficient heat transfer characteristics than can be achieved with the same streams arranged in series.
 2. The method of claim 1 wherein the first working fluid and the second working fluid have the same composition.
 3. The method of claim 1 where the first working fluid and the second working fluid are of different composition.
 4. The method of claim 1 wherein more than two working fluid circulation pathways are provided in the interior (tube side) of the exchanger.
 5. A method of construction of a heat exchanger that comprises providing a plurality of circulation paths in a plate finned and tube or finned tube exchanger utilizing a plurality of tube side circuits that are interlaced to accomplish more effective heat transfer than would be possible with a plurality of tube side fluid streams arranged in series without interlacing the circuitry.
 6. The method of claim 4 wherein the first working fluid and the second working fluid have the same composition.
 7. The method of claim 4 where the tube working fluids are of different composition.
 8. An energy recovery apparatus that comprises: a finned surface heat exchanger comprising a plurality of circulation pathways for a plurality of working fluids, and circulation means to pass a plurality of working fluids into at least one heat exchange zone.
 9. The apparatus of claim 8 wherein the apparatus comprises a plurality working fluid streams in circulation pathways of a plate fin and tube or finned tube exchanger.
 10. The apparatus of claim 9 further comprising multiple heat recovery stages to provide additional heat recovery.
 11. A method for designing an energy recovery system for increasing the efficiency of a gas turbine exhaust heat recovery by providing an integrated tube side heating circuitry to heat a plurality of working fluid circuits while cooling the exhaust stream of the gas turbine. 